Add --download-data-only option to run-test.py

Add --download-data-only option to run-test.py

The simd.js benchmarks reside in the same repository
that holds the compliance tests.

Adding an option to run-test.py to download test data
and exit, so that a recipe to run the simd.js
benchmarks can call run-test.py with this option
to fetch the test data.

BUG=https://code.google.com/p/v8/issues/detail?id=4124
LOG=N
TEST=manual verification it works.
NOTRY=true
R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/1140353002

Cr-Commit-Position: refs/heads/master@{#28446}